1. Technical Field
The present invention relates to a method and apparatus for laminating boards.
2. Description of Related Art
DVDs (digital video disks) have been developed as an information recording medium that makes use of light. The DVDs, which are capable of recording large amounts of data with far higher densities, compared with ordinary CDs, CD-ROMs, LDs and the like, are commonly regarded as suitable for high-quality long-time recording of images such as movies as well as high-volume storage of computer data.
The structure of DVDs is exemplified by two-layer lamination single-side-reading disks. Specifically, this type of DVD has a structure as shown in FIG. 21. Single boards A, B, in each of which a signal recording layer 1 made of a metal thin film or the like is sandwiched integrally between a disk body 2 and a protective layer 3 both made of transparent resin, are laminated together with an adhesive layer C interposed therebetween. By applying a laser beam L from one surface side and by changing its focal point, a signal can be read from either one of the two signal recording layers 1. In this structure, the recording capacity doubles that of a single-layer signal recording layer 1, and yet the laser beam L only needs to be applied from one surface side without the need of reversing the disk. Thus the structure has an advantage of simplified laser mechanism and disk handling mechanism.
For the production of such DVDs, it is necessary to perform the step of laminating previously and separately fabricated single boards A, B with the adhesive layer C.
A conventional method for laminating the single boards A, B is that, with a relatively large amount of adhesive applied on the entire surface of one single board, the other single board is placed on top of the coated board and then the two single boards are pressed together thus accomplishing the joint of single boards by adhesive.
For the single-side-reading system DVDs, the laser beam L applied from one surface side needs to serve for reading the opposite-side signal recording layer 1 through the adhesive layer C. Therefore, such defects as microscopic foams or faulty adhesion, if present in the adhesive layer C, would largely affect the information reading precision.
Unfortunately, with the conventional method for laminating single boards A, B as described above, there has been a tendency that foams enter the adhesive layer C, resulting in poor yields of products. It could be presumed that, during the process of placing the other single board B on the adhesive layer C that has been applied onto one single board A, local voids or pits and projections would inevitably be generated between the surface of the adhesive layer C and the opposing surface of the single board B, thus allowing foams to enter therein. Indeed increasing the amount of the adhesive to be applied to the single board would reduce the generation of foams to some extent, but it has been difficult to eliminate the foams at such high precision as is required for the DVDs. Still, using larger quantities of adhesive than is necessary for adhesion would result in a waste of adhesive and would moreover add to the work of treating the adhesive that has overflowed from between the single boards.
The aforementioned issue can occur, without being limited to the DVDs, also when various types of boards are laminated together by the medium of adhesive.